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Charles Darwin and His Theory of Natural Selection

 In 1838, a prolific writer compiled a list of pros and cons to decide whether to “marry” or “not marry”.1 Some of the pros for marriage included having children and having a constant companion (and a friend in old age), someone he could love and talk to. But bachelorhood also offered some compelling benefits like having the freedom to go anywhere and having more free time in general. With that freedom, the writer reasoned, he would be able to enjoy intelligent conversations with “clever men at clubs.” That writer was Charles Darwin, the father of evolutionary biology.

Charles Darwin (Courtesy: Wikipedia)

Charles Robert Darwin was born in Shrewsbury, Shrewsbury, England on 12 February 1809. He was the fifth child of Robert Darwin, a doctor, and Susannah Darwin. In 1825, he worked as an apprentice doctor under his father’s supervision and eventually was sent to the University of Edinburgh Medical School. He, however, neglected his studies because the medical lectures bored him and surgery distressed him. Instead, he spent time investigating marine animals.

Annoyed by Darwin’s lack of interest in medicine, his father decided to make him a parson. Since a college degree was necessary for that vocation, he sent Darwin to Christ’s College, Cambridge in 1827 to study for Bachelor of Arts. Darwin was happy there: he spent his spare time shooting birds and riding. He became interested in insects and began his beetle collection. He even managed to publish his findings in the Illustrations of British Entomology. Gradually, he became acquainted with several naturalists, including a botany professor named Joh Stevens Henslow. In 1831, Darwin passed his final exam with flying colors.

Soon after graduation, Henslow recommended him for an unpaid naturalist position on HMS Beagle. The ship would sail across the world carrying out hydrographic surveys and the naturalist on board would observe, record, and collect whatever specimens of animals, plants, rocks, and fossils he could find. Darwin was ecstatic; his father, less so. Ultimately, however, his father agreed to fund him for the voyage, and right after Christmas, on December 27, 1831, HMS Beagle set sail in the Atlantic Ocean with Darwin on board. The voyage was supposed to last only for a couple of years, but it ended up lasting for 5 long years. And that voyage would not only deeply affect Darwin personally afterward but also would revolutionize biology.

Whenever the ship came ashore, Darwin would leave the ship to explore the land. He kept a detailed log of pretty much everything that caught his attention: the scientific journal he kept covered biology, geology, and anthropology. 

Darwin's finches (Courtesy: Wikipedia)

During his voyage, Darwin went to the Galapagos Islands, located about 600 miles off the coast of Ecuador. As he surveyed several islands, he was struck by the way certain animals on one island differed from the same animal on a different island. For example, the beak shapes in finches varied considerably from one island to another. Cactus finches had longer, more pointed beaks than the ground finches, but the warbler finches had thinner and pointier beaks than both. 

Once back in England, Darwin spent time wondering about how those finches got different beaks. It seemed likely that they all came from a common ancestor boasting a particular beak, but over time, the beak shape in the descendants diverged. Eventually, Darwin came up with a theory that could explain how that could have happened, but he didn’t publish anything on this theory for a long time. 

By 1858—over twenty years after Darwin’s voyage ended—another British naturalist named Alfred Russell Wallace came up with a similar theory. After carefully observing the local fauna in modern-day Indonesia, Wallace concluded that that the animals evolve and that they evolve by adapting to their environment. He wrote a paper on it and sent it to Darwin for feedback. Struck by the similarities between their ideas, Darwin decided to act. He wrote a joint paper with Wallace arguing the theory of evolution and natural selection in the same year (more on this shortly). The following year, in 1859, Darwin published his most famous book: On the Origin of Species.2 Despite the innocuous title, it took both the scientific and non-scientific community by storm. 

What did Darwin mean in this theory of natural selection? As I alluded earlier when I discussed finches, the theory has two main pillars: ecology and hereditary.3 Darwin made the following ecological observations:

1. Within a habitat, the number of a particular organism has the potential to increase exponentially.

2. However, that doesn’t happen: the number remains mostly stable.

3. A habitat has limited resources (food, for example). 

Inference 1: Since the resources are limited, not all offspring can survive and reproduce.

And these were his hereditary observations:

1. Individuals are not identical: they differ in several characteristics.

2. The characteristics are heritable—that is, those characteristics can be passed to the offspring

Inference 2: Some individuals with particular traits are most likely to survive and reproduce than others.

Darwin ingeniously combined those two inferences to come up with his theory of natural selection. The theory of evolution by natural selection states that differences in survival and reproduction are not random and that some suitable traits are passed on at a higher rate than others, which increases the proportion of that trait in population from one generation to the next. These suitable traits are, thus, selected by nature, upon which the characteristics of that population will evolve.  

So how does the theory explain the difference in beak shapes Darwin noticed in the finches? The finches may have come from the same ancestor (from mainland Ecuador, for example), but not all the ancestors had identical beaks. Once the islands separated from the mainland and each other, those differences became important. For example, if some finches with more pointed beak could access nectar—the only food source available on an island—while others could not, only those finches will be able to survive and pass on the trait for the pointy beaks to their offspring. Over time, the finches on that particular island would only boast pointed beaks as finches with relatively stubby beaks will not be able to survive to pass on their traits to their offspring. The finch population, thus, will evolve to have pointed beaks over time, shaped by natural selection. 

I will cover more on evolution in later essays, but for now, let’s get back to Darwin. Despite bouts of debilitating illness, Darwin wrote at a great pace and penned several more books—both scientific and non-scientific—as well as worked on several editions of On the Origin of Species after his voyage. And then, on 19 April 1882, at the age of 73, Charles Darwin died at his home in London. In his last words to his wife Emma—yes, he did marry in 1839—he mentioned that he was “not the least afraid of death”.

1. https://www.darwinproject.ac.uk/tags/about-darwin/family-life/darwin-marriage

2. The full name of the book is On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life

3. https://evolution-outreach.biomedcentral.com/articles/10.1007/s12052-009-0128-1


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